The present invention relates to a method for measuring the extent of slag deposit buildup in the channel of a channel induction furnace.
A typical channel induction furnace used for melting metals comprises a container for holding molten metal and a U-shaped channel in communication with the container through two vertically spaced apart openings in the container wall and forming a loop path for the molten metal. Heating of the metal in such a furnace is accomplished by inductively coupling an electrical current in the metal in the loop path to provide resistance heating of the metal in the channel and to cause a convection flow of heated metal from the channel into the container. A problem with channel conduction furnaces is that slag in the molten metal tends to deposit on the walls of the channel near the openings thereof. Such slag deposits tend to restrict the convection flow of molten metal through the channel and thus reduce the heat transfer between the channel and the container. If the slag deposits are permitted to buildup sufficiently so as to cause significant blockage of the channel, heating of the metal in the container may become inadequate for maintaining the metal at a desired operating temperaure while the metal in the channel may become so overheated that the refractory lining of the channel is damaged causing leakage of molten metal to occur. Therefore, slag deposits in the channel of the channel induction furnace must be detected and removed before such blockage occurs.
One technique for detecting and removing the slag deposits is to visually inspect the channel after the furnace has been emptied and cooled down and to manually remove any slag deposits. However this technique is unsatisfactory inasmuch as cooling of the furnace tends to produce cracks in the furnace walls and thus unacceptably shortens the life of the furnace.
An improved technique for removing slag deposits which does not require cooling the furnace is disclosed in commonly assigned Japanese Patent Application No. 136515-1980, filed Sept. 30, 1980. With the improved technique slag deposits which are within certain limits of buildup may be removed by temporarily increasing the induction heating of the channel above that which is necessary to maintain the furnace at a desired operating temperature. The increased channel heating causes softening of the slag deposits and a strong convection flow of molten metal in the channel resulting in rapid erosion of the slag deposits. However, in order to use the improved slag removal technique, the extent of slag deposit buildup in the channel must be precisely measured while the furnace is in operation in order to permit a determination of the start and the duration of the increased channel heating necessary for slag removal without overheating the channel.
Heretofore known methods for measuring the extent of slag deposit buildup in the channel of an operating furnace include: measuring the change in power factor in the channel induction heating unit caused by a deterioration of the channel lining due to blockage and overheating of the channel; and measuring the increase in channel temperature resulting from blockage of the channel by slag deposits by an appropriate temperature sensing means such as a thermocouple or an optical pyrometer. However, these known methods are deficient for the purpose of controlling slag removal by increased induction heating owing primarily to a lack of precision in the measurements of the extent of slag deposit buildup provided thereby. In the power factor measurement method it is difficult to establish a precise relationship between the deterioration of the channel lining and the extent of slag deposit buildup. Moreover, it is desirable to remove the slag deposits before any deterioration in the channel lining takes place. In the channel temperature measurement method it is again difficult to precisely relate a rise in the channel temperature to the extent of slag deposit buildup. Therefore, a need clearly exists for a method for precisely measuring the extent of slag deposit buildup in the channel of an operating channel induction furnace.